Fiber reinforced structures of a variety of shapes are formed by drawing fibers coated with a thermosetting resin through one or more shaping dies, then curing the resin.
In U.S. Pat. No. 2,684,318 to Meek incorporated herein by reference for instance, glass fibers are fed from a spool through a gathering annulus, drawn through a liquid bath of a thermosetting resin, then gathered into a rod-like bundle, and pressure applied to squeeze out air entrapped between the gathered fibers to compress then into a rod-like bundle of predetermined cross-sectional shape and size, and passing the bundle under tension through a curing oven. Following cure, the rods are cut into desired lengths.
In U.S. Pat. No. 2,871,911, to Goldsworthy et al, also incorporated herein by reference, pertains to a similar pultrusion process where dielectric heating is utilized to effect a cure.
In U.S. Pat. No. 2,948,649 to Pancherz, and incorporated herein by reference, the fibers in resin are passed through a molding tube where the fiber and resin and plastic material are heated to a point where the resin is on the borderline of passing from a rapidly flowing state into a gel state. The structure is then cooled during the remaining part of its travel through the shaping tube in order to interrupt the course of gelling and hardening of the plastic on the outer layer adjacent the peripheral surface thereof, but to permit the resin to proceed to hardening closer to the central axis of the tube. The structure emerging from the tube is again heated to harden the plastic at the outer layer and finish the hardened plastic surface.
In U.S. Pat. No. 3,977,630 to Bazler, incorporated herein by reference, there is in substance described another general method for forming structures of duct-like shape.
Yet another method of preparing fiber reinforced structures is described in U.S. Pat. No. 3,718,449 to Fahey, incorporated herein by reference.
In my U.S. Pat. No. 4,168,194 incorporated herein by reference, there is described a pultrusion method where continuous fibers, coated with the heat curable thermosetting resin composition below the temperature at which cure is initiated, are passed through a plurality of in-series elongated open heating zones, each having at least one internal heating surface spaced from each other, in between which are imposed at least one cold shaping die which is relatively narrow with respect to length of the heating zone, and having a structural shaping orifice. The internal surfaces of the heating zone are in spaced relation to the resin coated fibers such that the resin is heated by radiation and convection to a temperature sufficient to reduce viscosity of the resin relative to the introduction viscosity of the resin to the heating zone and initiate partial cure of the resin. The heated resin is then drawn through the orifice of the cold shaping dies between each heating zone at a die temperature maintained substantially below the temperature at which the curing of the resin is initiated. Following the last of the cold shaping dies, the structure at the gel point is passed through a final heating zone, termed a "curing zone", to promote the exothermic curing reaction in the hope of achieving complete resin cure.
I have found that in the practice of such a pultrusion process or other structure forming process that passage through an open final curing zone is incapable of achieving under practical operating conditions complete cure, i.e., substantially complete cross-linking and/or polymerization of the resin. This has been found to be true, even though the surface appears hard. Cure can, therefore, continue when the structure is in storage, whether cut to length or stored on a reel. In the former case, the structure can change shape, as by sagging. In the latter case, the structure can change shape or take a "set" conforming to the diameter of the take-up reel. In either case, this can induce a "memory" which is undesirable in subsequent handling of the finished structure.
The avoidance of this situation is exemplified by the highly refined structure described in my U.S. Pat. No. 4,113,349, incorporated herein by reference. The ability to relieve stresses on the optical fiber during manufacture can be offset if the structure takes, during extension of cure, the shape of the take-up reel in which instance the final structure is difficult to uncoil and lay flat.
Curing of resin coated fibers in an open heated curing zone exposed to air may increase the rate of cure but to the detriment of the resin, degrading at least the surface and materially darkening the surface.
A need, therefore, exists for a modification of the above-identified processes to insure that cure is complete, or essentially complete, before the structure is cut to length or taken up on a storage reel or the like.